This invention relates to a two-pole stator winding for a dynamoelectric induction machine and particularly a polyphase induction motor.
Alternating current induction motors have been developed as suitable power driving sources. Polyphase motors and particularly three phase motors are widely applied in industrial and similar heavy duty applications. A rotor is rotatably mounted within an annular stator. The stator is wound with a three phase winding connected to a three phase alternating current power supply. The rotor is normally provided with a short circuited winding within which responds to the stator field to create an induced field. The power supply includes phase voltages and currents which are offset in time by a predetermined angular displacement. A three phase power supply has three voltages and currents which are offset from each other by 120 electrical degrees. In the three phase motor, three distinct phase windings are provided one for each power phase of the power supply. The three phase winding thereby develops a magnetic field which moves circumferentially about the stator and rotor. The induced field tends to align with and follow the rotating field to create a rotating force and motion of the rotor as a result of the electromagnetic coupling between the fields of the stator and the rotor. Each phase winding is conventionally formed of a plurality of individual coil groups, with each group of phase coils generally distributed as a part of the phase winding. In a two pole machine, coil pairs are provided circumferentially spaced from each other by 180 degrees. The coils are wound to form complementing poles spaced approximately 180 electrical degrees from each other.
The windings are wound of copper or other low resistance wire. When connected to the conventional three phase power supply, a high level current flows through the winding with the rotor stationary or rotating slowly. Under motor running operation, the current is limited by the electromagnetic coupling of the stator field and the induced field of the rotor in accordance with well known electromagnetic phenomenon. At and during the starting of the motor, the rotor is at an essential standstill and accelerates relatively slowly from standstill to the operating speed. During this short period of starting and acceleration, the winding is a low resistance wire load and high level currents would flow through the coils, in the absence of special compensation systems connected into the winding circuit. Although the current surges encountered may not seriously damage the motor, such surges are reflected by the power supply back to the system to which the motor is connected. An effect which is often encountered is a momentary dimming of lights connected to the same power supply system, and which is generally considered at best to be an undesired annoyance.
The prior art discloses various circuit connections to minimize and limit the starting current in order to minimize current surges on the supply.
Generally, the special winding connections are connected to a suitable switching circuit to limit the current, to operate the motor with special circuits operating at a relatively low efficiency until such time when the circuit is switched to establish the running circuit connection.
A highly satisfactory induction motor employs a stator winding of a double-delta connected configuration. In the double delta winding circuit, the three phase windings are interconnected to form an equi-triangle with a phase winding in each leg. The power supply is connected to the apexes or points of the triangle. In the double delta circuit connection, coil pairs in each leg are connected in parallel for generating the corresponding north and south poles. A conventional manner of limiting the current during starting is to reconfigure the phase winding during starting with the pairs of windings of each phase connected in a series circuit with each other and with the winding of another phase. It is well known that when a normal phase relationship between the current in the several windings is disrupted that the voltage is reduced on all coils. Thus, the impedance, that is, the resistance to alternating current flow of the stator winding, is much higher and significantly minimizes the starting current surges.
U.S. Pat. No. 4,675,591 to Bernard J. Pleiss and assigned to a common assignee with this application, for example discloses a special four-pole (or multiples thereof) motor with a special bi-filer chain wound winding with the phase windings connected in a delta circuit. The Pleiss patent discloses the special chain winding concentrically wound for winding a machine with four poles and higher multiplexes of four poles. The special two layer chain winding provides ease of winding and assembly. To start the motor, the winding is connected in a series delta circuit connection and switched to a parallel delta circuit for running.
Although relatively widely used and a highly satisfactory circuit, the conventional two pole motor will not reach operating speed in the start circuit connection. Although other winding configurations may reach operating speed, such alternate configurations are complex or do not provide an adequate reduction or limit on the starting current.
In a typical polyphase or two pole, three phase motor, the stator is formed with a substantial plurality of circumferentially spaced winding slots. Each coil of the coil pair of a phase winding includes a plurality of coils. The coils are generally prewound with the sides of different circumferential spacing generally by and differing by a coil slot, such that a given phase winding consists of a plurality of immediately adjacent individual coils interconnected at the opposite axial ends to form a series of coils for establishing the corresponding plurality or pole. In a conventional winding practice, each slot will include the sides of different phase coils to permit the compact and appropriate locations of the poles to establish the interaction between the stator phase windings and the field of the rotor.
For example, U.S. Pat. No. 4,492,890 discloses a stator winding configuration for controlling the current flow and magnetic field created by the coils. It is specially constructed to provide a high efficiency operation. That patent discloses a method of special coil distribution within the slots to effect the improvement in motor operation. As more fully discussed therein, the chordal effect of the coil configurations will modify the harmonic content to establish and maintain a highly efficient motor operation. Such a solution however requires a special coil with special orientation and insertion into the coil slots.
In a conventional practice, the preformed coils are hand placed in the slots or placed on a supporting fixture which is introduced into the stator and expanded outwardly to place the windings into the appropriate slots. The use of the different number of turns of the coil and the modified distribution of the coil sides, such as typical of the U.S. Pat. No. 4,492,890, may require special attention and consideration in forming of the stator. Further, it is not clear that the motor will start even with such winding distribution and particularly reach operating speed under the starting circuit connection.
There is therefore a need for a winding configuration and system and particularly in a two pole motor which will cause the motor to accelerate to operating speed on the start winding circuit connection with a significant reduction in locked rotor current to prevent severe current surges on the power supply and permit switching only after or essentially reaching operating speed.